The present invention relates to an apparatus for detecting arrhythmias in electrocardiogram signals by evaluating the time interval between signals which succeed one another in time and which are characteristic for a heartbeat.
It is known to obtain information about the activity of a patient's heart by a real time evaluation of the appearance of signal components in the EKG. Since cardiac activity, however, in dependence on stress, medication etc. is subject to time-dependent changes, it is necessary, in order to obtain a precise diagnosis, to monitor the cardiac behavior over a long period of time, in some cases several days.
In order for the physician or his assistant not to be required to continuously visually follow the curves of the electrocardiogram, automatically operating histogram stores, or memories, have been developed which determine the time elapsed between successive heartbeats and store a representation of that time in counters in separate classified arrangements so that at the end of the monitoring period an overview of the statistical distribution of the heartbeat rates is available.
However, with this method it is not possible to obtain information about the dynamics of the heartbeat behavior, i.e. to indicate how the heart rate has changed in the course of time. For example, in the case of a particular kind of change, it can not be determined whether, over a relatively long period of time, a patient had bradycardia as well as tachycardia or experienced double beat phenomena, i.e. an alternation between two heartbeat rates, from one to the other.
Attempts have been made in this connection to obtain information about changes in cardiac activity from one heartbeat to the next by recording so-called difference histograms in that the times between two successive heartbeats are compared. If the time deviation exceeds a given difference value, a signal indicating an arrhythmia is emitted. In this case the differences in the times between two successive heartbeats are also classified as to time and are stored in different counters so that at the end of the monitoring period the statistical distribution of the differences in times between two successive heartbeats is available.
This solution has the drawback that the determined absolute time differences permit to accurate conclusion regarding the patient's cardiac behavior. The determined and recorded time differences exhibit a much greater value for coinciding determinations at slow heartbeat rates than at fast heartbeat rates so that erroneous interpretations cannot be excluded. During recording over a long period of time with the aid of a histogram store, subsequent association of the time differences recorded with the aid of counters with the different heartbeat rates that have occurred is not possible.
U.S. Pat. No. 3,755,783 discloses an analyzer for biomedical signals in which irregularities in the cardiac rhythm are determined by classifying the signals characteristic for heartbeats with respect to the time elapsed since the respective previous heartbeat. Since in this classification all time periods which serve as a standard are fixed, these must always be selected to be lower than the shortest time period between two successive regular heartbeats to be expected. However, at a low heart rate, a heartbeat following at a time interval which for a high heart rate would still be regular already indicates an arrhythmia. Therefore, this prior art analyzer cannot detect all irregularities in a heart rhythm.